This invention is in the field of adhesives, and more particularly, acrylic pressure sensitive adhesives which have improved bonding characteristics to low energy surfaces.
As used herein, the term xe2x80x9cfilmxe2x80x9d refers to a thin, flexible, single- or multi-layer polymeric sheet. The term is used interchangeably with the terms xe2x80x9cbackingxe2x80x9d and xe2x80x9ccarrier webxe2x80x9d. Graphic marking films or labels formed from vinyl films coated with acrylic pressure sensitive adhesives (PSAs) arc well-known in the art. However, to date, the ability to provide such label and graphic marking film films with high bonding strength, (i.e. non-removal), to low energy surfaces such as high density polyethylene and polypropylene plastics has not been possible without sacrificing important properties of the adhesive such as shear strength and cohesive strength.
As used herein, the term xe2x80x9clow energy surfacesxe2x80x9d is intended to mean those surfaces which exhibit low polarity and low critical surface tension (less than about 40 dynes/cm2) characteristics. One example of a low energy surface is the surface of a polyolefin plastic. Among the PSAs, it is known that acrylic-based PSAs exhibit poorer bond characteristics to low surface energy polyolefin plastics than do rubber-based PSAs. This effect results from the greater difference in polarity between the acrylic PSA and the polyolefin surface as compared to that between the rubber PSA and the polyolefin surface. Unfortunately, however, it would be very desirable to use acrylic PSAs in many applications, since acrylic PSAs exhibit excellent outdoor durability, whereas rubber PSAs show poor ultraviolet and oxidative stability due to chemical unsaturation of the hydrocarbon elastomer.
Common physical methods to obtain high bond strength of pressure sensitive adhesives to polyolefin plastics include flame treating, which oxidizes the surface of the plastic, chemical etching with strong acids to increase polarity of the bonding surface, or the use of a primer or topcoat containing a chlorinated polyolefin. For example, Japanese Patent No. HEI 1(1989)-242676 discusses the use of chlorinated polyolefin resins in connection with pressure sensitive adhesives. One disadvantage of such surface treatment methods that they are inefficient in that they add an additional process step when applying graphic marking films or labels to low energy surfaces. This is less than ideal since industrial consumers of graphic marking films and labels desire that these products can be easily applied in a single step without the need for additional, time consuming surface preparation methods.
For a permanent graphic marking film or label application, it would be desirable to have bond characteristics similar to those associated with rubber-based PSAs combined with the outdoor durability associated with acrylic PSAs. One method to increase bond strength of acrylic PSAs to low surface energy polyolefin plastics is to incorporate a compatible tackifier such as a rosin ester, a terpene phenolic resin or a hydrocarbon resin into the adhesive. Although the use of a tackifier dramatically improves bond strength as measured by peel force at low speeds, inclusion of these tackifiers raises the glass transition temperature (Tg) of the PSA which results in reduced low temperature performance and also causes a xe2x80x9cshockyxe2x80x9d or xe2x80x9czippyxe2x80x9d peel characteristic at faster peel rates. This xe2x80x9cshockyxe2x80x9d or xe2x80x9czippyxe2x80x9d peel is an undesirable characteristic which can result in easy removal of films and labels, as well as making them less tamper-resistant.
Additionally, loss in adhesive shear strength and cohesive strength is also observed if large amounts of tackifier or plasticizer are incorporated into the PSA.
In many applications, it is desirable that graphic marking films or labels be difficult to remove once they are applied to a surface. This may be accomplished by providing the graphic marking film or label film with a means by which attempts to remove it will result in tearing or other damage to the graphic marking film or label. A method to increase the destructibility of PSA coated vinyl labels or graphic marking films is to make the vinyl film backing less elastic or xe2x80x9cbrittlexe2x80x9d. This is achieved by adding a hard acrylic resin and decreasing the plasticizer level of the vinyl film.
U.S. Pat. No. 5,141,790 (Calhoun et al.) and U.S. Pat. No. 5,296,277 (Wilson et al.) the teachings of both of which are incorporated herein by reference, describe an adhesive film referred to commercially as Controltac Plus(trademark) film (available from Minnesota Mining and Manufacturing Company, hereafter xe2x80x9c3Mxe2x80x9d). The adhesive surface of the Controltac Plus(trademark) film is characterized in that it includes clustered domains of a non-adhesive material, referred to as xe2x80x9cpegsxe2x80x9d which extend a short distance from the adhesive surface. The patents also describe adhesive films in which the adhesive surface is microtextured or provided with a microtopological structure.
The present invention relates to the incorporation of a plasticizer material into a tackified acrylic PSA. The resulting PSA is shown to provide improved bonding of polyvinylchloride graphic marking films and labels to low energy surfaces such as high density polyethylene plastic.
More specifically, the invention relates to tackified and plasticized acrylic PSA compositions comprising:
a) about 100 parts by weight of an acrylic copolymer, said acrylic copolymer comprising from about 70-98% by weight of one or more monofunctional acrylates having nontertiary alkyl groups with between 1 and 14 carbon atoms and from about 30-2% by weight of a polar monomer;
b) about 10-40 parts by weight of a tackifier,
c) about 3-10 parts by weight of a plasticizer; and
d) optionally, a crosslinker.
The invention also relates to films which incorporate such tackified adhesives, and the use of those films as graphic marking films and labels.
Specific chemical classes of plasticizers, when incorporated into the tackified pressure sensitive acrylic adhesive, improve the bonding characteristics of the adhesive to low energy surfaces. The addition of plasticizer to a tackified acrylic adhesive has been found to improve the wet-out or xe2x80x9cquick stickxe2x80x9d to low energy surfaces. The addition of plasticizer offsets the increase in glass transition temperature caused by the tackifier and improves bond strength. When incorporated at levels of less than about 10 parts plasticizer per 100 parts adhesive, the presence of the plasticizer results in little effect on shear performance or cohesive strength of the adhesive.
Among the plasticizers discovered to improve bonding characteristics are polyglycol ethers, polyethylene oxides, phosphate esters, aliphatic carboxylic acid esters, benzoic esters, and combinations thereof. In addition, other plasticizers which improve bonding to low energy surfaces have been identified, however, these have been found to cause some decrease in cohesive strength. These include sulfonamides and aromatic carboxylic acid esters.
The incorporation of the specified plasticizers along with acrylic-compatible tackifiers allows one to construct an outdoor durable vinyl label or graphic marking film with improved adhesion to low energy surfaces, and in particular, high density polyethylene surfaces, without the need for physical or chemical treatment of the surfaces. In one embodiment, the label or graphic marking film construction may include a destructible vinyl film backing for vandal-resistant applications. In this embodiment, the graphic marking film or label can be die cut with a microperforated or micro-rough steel rule which will initiate tearing when removal is attempted. Removal is further deterred by the improved adhesion characteristics of the graphic marking film or label. Despite these modifications to the film, the article maintains flexibility and fit strength for easy fabrication in graphic marking film or label manufacture.